The present invention relates to gas sensor diagnosing method for examining whether a gas sensor disposed in an exhaust passage of an internal combustion engine is in an improper condition or not, and/or gas sensor control or diagnostic apparatus for such a gas sensor.
A three-way catalyst is widely used in internal combustion engines for motor vehicles and other applications, to purify CO, HC and NOx contained in exhaust gas mixture. Furthermore, in order to improve the efficiency of the purification, there is provided a feedback air-fuel ratio control system including a gas sensor disposed in the exhaust passage of the engine, for sensing the oxygen concentration in the exhaust gases, and a controller for controlling the mixture ratio or air-fuel ratio toward the theoretical air-fuel ratio by regulating a fuel supply quantity, such as a fuel injection quantity, to the engine in accordance with the sensed oxygen concentration. Recently, the feedback air-fuel ratio control system often employs, as the gas sensor, a wide-range or full-range air-fuel ratio sensor varying the sensor output linearly in accordance with the oxygen concentration in the exhaust gases, in order to improve the accuracy of the feedback control.
Since the sensing element of the gas sensor is exposed directly to the exhaust gases in the exhaust passage, a poisoning component, such as phosphorous, in the exhaust gases can adhere to porous portions through which the exhaust gas mixture is introduced into the inside of the sensing element. If the amount of adhesion of the poisoning component increases too much, the sensitivity of the gas sensor (the gain of the sensor output) with respect to a change in the concentration of the specified component might decrease. Moreover, if cracks are formed in the sensing element, the sensitivity of the gas sensor might increase too much beyond the normal level. If the sensitivity of the gas sensor is not normal, the feedback air-fuel ratio control system might become unable to control the air-fuel ratio normally, and the three-way catalyst might become unable to purify the harmful components sufficiently.
Therefore, published Japanese Patent Applications, Publication Numbers S60-233343 and S60-192847 propose a gas sensor diagnostic method for detecting degradation of a gas sensor. While the fuel supply to an internal combustion engine is being interrupted, the air of the atmosphere is supplied to the gas sensor, so that a value of the sensor output is predictable. By utilizing the fuel supply interruption period, the diagnostic method determines the existence or nonexistence of a malfunction in the feedback air-fuel ratio control system inclusive of the gas sensor, by comparing a gas sensor output value obtained a predetermined time after a start of the fuel supply interruption, with a predetermined reference value. The feedback air-fuel ratio control system can perform this diagnostic method with its own controller without the need for a special sensor.